[ranxianglei]

What changes

shouldReserveOutputHeadroom() now returns true for every wire protocol — the Anthropic exemption is dropped. Anthropic-wire sessions reserve min(max_tokens, 25% × window) from the context window before nudge/truncate bands are computed, same as OpenAI-family wires already did.

Why

Decision made by the owner on 2026-10-21 after #3900's v0.1.203 verification: an over-full context costs attention everywhere, so paying up to a quarter of the window in reservation on Claude routes is worth removing the "provider counts completion against the window" class of 400s.

• [BUG] acp-proxy upstream HTTP 400: Request with 1,198,269 tokens exceeds the model's 1,048,576 token limit, consistently | [BUG] acp-proxy upstream HTTP 400：请求 1,198,269 tokens 超过模型上限 1,048,576，一直复现——是插件压缩未生效，还是 DSH 的问题？ #3900's 400 came from DeepSeek behind an Anthropic facade: 1,070,269 + 128,000 > 1,048,576. That provider counts output against the wall; real Claude does not. The old code decided by protocol string, so the wrapper inherited Claude's exemption and could blow through its wall on the first shot of every new baseline.
• After this change no wire can ship content+output past its own declared window on first contact. Residual overshoot only exists if the requested max_tokens itself exceeds the 25% cap — and that case always existed on every other wire too (self-heal handles one overflow turn).

Tradeoff (accepted)

Real-Claude routes lose up to min(max_output, 25%) of usable history until #3973 lands per-provider wall facts and can skip the reservation where the input limit is provably independent. That is #3973's optimization space, not this PR's precondition.

Deliberately not done

• No new config surface — the toggle was behavioral (src/util.ts); call sites (src/server/handle.ts, src/server/side-request.ts) unchanged, they just get true from the gate now.
• #3973 untouched and still needed for the reverse direction (reclaiming the band on genuine Claude routes).

Tests

• tests/context-overflow.test.ts: gate test re-pinned ("every protocol reserves").
• tests/preflight-max-output-tokens.test.ts: the anthropic e2e (fix: Codex preflight summaries fail with unsupported max_output_tokens #663) ran knife-edge against the old no-reservation wall (+2,000 context restores its documented margin; 12_000 − 1_024 ≈ 10_000).
• tests/anthropic-beta-window.test.ts: expected effective limits now include the reservation (1M−1024, 200K−1024).
• Local: 81/81 on the 7 core headroom/preflight suites, 58 pass / 0 fail across all 14 anthropic-named suites, tsc --noEmit clean. Full green per remote CI as usual.

Refs #3900 #3973

[github-actions]

📦 Merged Build Published

This merge to master was published as 0.0.725-master.ef59cf7 (rolling master tag).

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[github-actions]

📦 Built Package Artifact

Branch: 2026-10-21_always-reserve-headroom (4d50f9c)

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